Capacitor cell supporting circuit operation at higher-voltages while employing capacitors designed for lower voltages

ABSTRACT

An integrated circuit (IC) includes a functional circuit and a capacitor cell. The functional circuit may operate with one of two power supply voltages. The capacitor cell is used to provide power supply decoupling for the functional circuit, and includes multiple capacitors, each designed to withstand a maximum voltage equal to the lower of the two power supply voltages. When the functional circuit is to operate with the higher of the two power supply voltages, the capacitors in the capacitor cell are coupled in a series arrangement between power supply and ground terminals of the IC. When the functional circuit is to operate with the lower of the two power supply voltages, the capacitors in the capacitor cell are coupled in a parallel arrangement between the power supply and ground terminals. In an embodiment, the functional circuit is an input-output (I/O) circuit powered by 1.8V or 3.3V power supplies.

BACKGROUND

1. Technical Field

Embodiments of the present disclosure relate generally to de-couplingcapacitors, and more specifically to a capacitor cell supporting circuitoperation at higher-voltages while employing capacitors designed forlower voltages.

2. Related Art

Both integrated circuit capacitors and conventional parallel platecapacitors have voltage rating specifying the maximum voltage that canbe applied across their terminals without causing damage to thecapacitor structure. Integrated circuit capacitors are often implementedusing a metal-oxide-semiconductor (MOS) transistor structure, fabricatedusing corresponding fabrication processes. Typically, the gate-oxidethickness is determined by the fabrication process, and may be designedto target a specific operational environment, such as for example,operation with low-voltage power supplies. One example of a MOScapacitor fabrication process is a 1.8V fabrication process, in whichthe gate oxide of the MOS capacitor is designed to withstand a maximumvoltage across it of 1.8V.

Capacitors often need to be employed in circuits that operate from powersupply voltages that are greater than the maximum safe voltage that canbe applied across the capacitors. An example is an input-output (I/O)circuit that can selectively be chosen to operate from a 1.8V powersupply and a 3.3V power supply, and which is power-supply-decoupled by adecoupling capacitor fabricated to withstand a maximum of only 1.8Vacross its terminals. In such circuits, it may need to be ensured thatthe use of capacitors designed for lower voltages can reliably supportcircuit operation at higher voltages, i.e., without damage to thecapacitors.

SUMMARY

This Summary is provided to comply with 37 C.F.R. §1.73, requiring asummary of the invention briefly indicating the nature and substance ofthe invention. It is submitted with the understanding that it will notbe used to interpret or limit the scope or meaning of the claims.

An integrated circuit (IC) includes a functional circuit and a capacitorcell. The functional circuit is coupled between a power supply terminaland a ground terminal of the IC. The functional circuit receives a powersupply for operation on the power supply terminal. The power supply isprovided with a first voltage value for a first logic-level swing of anoutput of the functional circuit, and with a second voltage value for asecond logic-level swing of the output of the functional circuit. Thefirst voltage value is greater than the second voltage value, and thefirst logic-level swing is greater than the second logic-level swing.The capacitor cell is coupled between the power supply terminal and theground terminal, and includes a first capacitor, a second capacitor, andmultiple switches. When the output is to be provided with the firstlogic-level swing, the switches are operable to couple the firstcapacitor and the second capacitor in a series arrangement between thepower supply terminal and the ground terminal. When the output is to beprovided with the second logic-level swing, the switches are operable tocouple the first capacitor and the second capacitor in a parallelarrangement between the power supply terminal and the ground terminal.

Several embodiments of the present disclosure are described below withreference to examples for illustration. It should be understood thatnumerous specific details, relationships, and methods are set forth toprovide a full understanding of the embodiments. One skilled in therelevant art, however, will readily recognize that the techniques can bepracticed without one or more of the specific details, or with othermethods, etc.

BRIEF DESCRIPTION OF THE VIEWS OF DRAWINGS

Example embodiments will be described with reference to the accompanyingdrawings briefly described below.

FIG. 1 is a block diagram illustrating the details of an examplecomponent in which several embodiments can be implemented.

FIG. 2 is a circuit of a capacitor cell in an embodiment.

FIG. 3A is an equivalent circuit of a capacitor cell in a mode ofoperation, in which a higher of two power supply voltages is used foroperation of I/O circuits of an IC.

FIG. 3B is an equivalent circuit of a capacitor cell in a mode ofoperation, in which a lower of two power supply voltages is used foroperation of I/O circuits of an IC.

The drawing in which an element first appears is indicated by theleftmost digit(s) in the corresponding reference number.

DETAILED DESCRIPTION

Various embodiments are described below with examples for illustration.

1. Example Component

FIG. 1 is a block diagram illustrating an example component in whichseveral embodiments of the present disclosure can be implemented.Integrated circuit (IC) 100 is shown containing processor 110,input-output (I/O) circuits 150-1 through 150-N, capacitor cells 160-1through 160-M, and voltage detector cell 170. Terminals 101, 140 and 145represent power supply terminals. In an embodiment, supply voltages onterminals 101 and 140 respectively equal 1.0V and 1.8V. In theembodiment, based on whether I/O circuits 150-1 through 150-N are togenerate output signals (185-1 through 185-N) with a logic-level swingof 0V-1.8V or 0V-3.3V, the supply voltage received on terminal 145equals 1.8V or 3.3V. Logic-level swing refers to the voltage differencebetween the logic-high and logic-low levels of the output signals.Terminal 199 is a ground terminal. Terminals 185-1 and 185-N representpads (or package pins) of IC 100.

While the specific values of power supply voltages on terminal 145 arenoted as 1.8V and 3.3V, the values can be different in otherembodiments. Similarly, the values of power supply voltages on terminals140 and 101 can be different in other embodiments. The details of FIG. 1are meant to be merely illustrative, and IC 100 may contain moreblocks/components and/or different arrangement of the blocks/components.

Processor 110 receives a power supply on path 101. Processor 110generates binary data on each of paths path 115-1 through 115-N. Thelogic-level swing of data on each of paths 115-1 through 115-N isdetermined by the value of power supply voltage 101, and equals 0V-1.0Vin an embodiment.

I/O circuit 150-1 receives binary data on path 115-1, and generates acorresponding buffered data on path/pad 185-1. The output impedance ofI/O circuit 150-1 may be designed to have a controlled value to matchthe characteristic impedance of the transmission path connected to pad185-1 to minimize signal reflections. Each of I/O circuits 150-2 (notshown) through 150-N operates in a manner similar to I/O circuit 150-1to generate corresponding buffered outputs based on the data received asinputs. Thus, I/O circuit 150-N receives binary data on path 115-N, andgenerates a corresponding buffered data on path/pad 185-N.

Based on whether I/O circuits 150-1 through 150-N are required togenerate output signals with a logic-level swing of 0V-1.8V or 0V-3.3V,the supply voltage received on terminal 145 respectively equals 1.8V or3.3V. In general, I/O circuits 150-1 through 150-N are designed togenerate output signals with one of two logic-level swings, with onelogic-level swing being greater than the other. Voltage detector cell170 receives the voltage at power supply terminal 145, and generatesmode signal 135 (mode) specifying whether the voltage on power supplyterminal 145 is 1.8V or 3.3V. Mode signal 135 may be a binary signal,with one logic level specifying that the voltage on path 145 equals 1.8Vand the other logic level specifying that the voltage on path 145 equals3.3V. I/O circuits 150-1 through 150-N, and voltage detector cell 170may be implemented in a known way. Typically, power supply 145 is usedto power the output stages of each of I/O circuits 150-1 through 150-N.An output stage of an I/O circuit generally refers to the portion of theI/O circuit that generates an output signal (such as the signal providedon pad 185-1). Power supplies 140 and 101 are used to powerlevel-shifter stages and other control blocks contained in each of I/Ocircuits 150-1 through 150-N.

Each of capacitor cells 160-1 through 160-M contains one or morecapacitors, and is designed to provide decoupling capacitance betweenthe power supply (145) and ground (199) terminals of each of respectiveI/O circuits 150-1 through 150-N. Although in FIG. 1, each I/O circuitis shown as being decoupled by a different (unique) capacitor cell, itis noted here that the number (M) of capacitor cells in IC 100 may bedifferent from the number (N) of I/O circuits that need to be provideddecoupling capacitance. Typically, M is smaller than N, and a singlecapacitor cell may be used to decouple more than one I/O circuit. Thespecific ratio M:N may be selected based on the signal-to-power (s2p)ratio for IC 100, as noted below.

As is well-known in the relevant arts, the maximum voltage that acapacitor can withstand across its terminals is usually limited. Avoltage greater than such maximum voltage may damage the capacitor. Whenimplemented within an IC, capacitors are usually implemented using a MOS(metal oxide semiconductor) transistor structure, referred to henceforthas a MOS capacitor. The characteristics (including thickness) of thegate oxide of such MOS transistor structure typically determine amaximum voltage that such capacitors can withstand across theirterminals (inter-terminal voltage) without damage.

Thus, for example, a MOS capacitor fabricated using a 1.8V gate-oxideprocess is typically able to withstand an inter-terminal voltage of amaximum of 1.8V. Similarly, a MOS capacitor fabricated using a 3.3Vgate-oxide process is typically able to withstand an inter-terminalvoltage of a maximum of 3.3V. Further, the capacitance of a MOScapacitor is generally a function of the voltage applied across itsterminals.

The value of decoupling capacitance provided for a circuit (such as anyof I/O circuits 150-1 through 150-N) may have a bearing on the signalintegrity (or signal quality) of the output signal of the circuit, aswell as the area required to implement the decoupling capacitor.Generally, larger the decoupling capacitance, better is the signalquality of the output signal, and larger the implementation arearequired for the capacitor.

In an IC (such as IC 100), decoupling capacitors are typicallyimplemented as part of power cells and ground cells of the IC, andplaced at desired (typically equal) intervals in an I/O ring formed byI/O cells (such as I/O circuits 150-1 through 150-N) of the IC. Theratio of the number of power cells (and ground cells) to the number ofI/O cells may be specified by a desired signal-to-power (s2p) ratio,determined in the IC design stage. Thus, for example, an s2p ratio of6:1:1 specifies that one power supply cell and one ground cell needs tobe implemented for every six I/O cells (or I/O circuits). However, ifdecoupling capacitance values realizable (or implementable) are notsufficiently high, the s2p ratio may need to be tightened, for example,to 4:1:1, i.e., one power supply cell and one ground cell for every fourI/O cells. Such tightened s2p requirement (due to relatively lowerdecoupling capacitance values realizable) typically translates to a needto implement a greater number of power and ground cells for a same totalnumber of I/O cells, thereby potentially resulting in largerimplementation area (i.e., larger die size).

A measure of “efficiency” of a decoupling capacitor is the ratio of thevalue of capacitance (and therefore the impedance that the decouplingcapacitor offers to noise in a target frequency range, a lower impedancetranslating to better efficiency) that is realizable to the arearequired for implementing the decoupling capacitor. The target frequencyrange with respect to decoupling capacitors may, in general, depend onseveral factors such as the current (and variations in the current)drawn by a load from the output of an I/O circuit, package and boardparasitics, power plane noise, ground plane noise, etc. In anembodiment, the target noise frequency range is 50 MHz to 150 MHz.

It may be desirable to implement a decoupling capacitance scheme tosupport circuit operation at higher-voltages while employing capacitorsdesigned for lower voltages. Further, it may be desirable to implement adecoupling capacitance scheme to maximize the capacitance obtainablewithout incurring an increased area penalty.

2. Capacitor Cell

FIG. 2 is a circuit diagram of a capacitor cell designed to providedecoupling capacitance, in an embodiment. Capacitor cell 160-1 of FIG. 1is shown containing capacitors 210 and 220, and transistors 230, 240,250, 260, 270 and 280. Bulk terminals of capacitors 230, 240, 260 and270 are connected to a voltage 201. Node 201 receives a voltage equal toVDDS during normal operation, but during power-up of IC 100 may receivea different bias voltage to bias the bulk terminals appropriately toprevent latch-up. Capacitors 210 and 220 are MOS capacitors, and areused to provide decoupling capacitance in each of a 3.3V mode (whenpower supply 145 equals 3.3V) and 1.8V mode (when power supply 145equals 1.8V) of operation of I/O circuit 150-1. Each of capacitors 210and 220 is fabricated using a 1.8V gate oxide process. Hence, themaximum safe voltage that can be applied across each of the capacitorsequals 1.8V.

It is assumed in the description below that the ON resistance of thetransistors 240, 250 and 260 of FIG. 2 is zero or negligible. Howeverthe ON resistance of transistor 230 is designed to have a large value.In an embodiment, power supply 140 (1.8V) is provided to each ofcapacitor cells 160-1 and 160-M since power supply 140 is required foroperation of I/O circuits 150-1 through 150-N, and thus readilyavailable. In embodiments in which power supply 140 is not required foroperation of the corresponding circuits for which decoupling capacitanceis to be provided, a voltage reference providing the same voltage aspower supply 140 may be used. Such a voltage reference may either beimplemented within the corresponding capacitor cell, outside thecapacitor cell but within IC 100, or be implemented external to IC 100.

Capacitor cells 160-2 through 160-M (of FIG. 1) are implemented similarto capacitor cell 160-1. The operation of capacitor cell 160-1 isdescribed next.

Operation in 3.3 V Mode

In the 3.3V mode, power supply at terminal 145 (Vdds) equals 3.3V, andpower supply at terminal 140 (Vdd) equals 1.8V. Mode signal 135generated by voltage detector cell 170 is a logic zero. As a result,transistors 230 and 240 are ON, and transistors 250 and 280 are OFF.Since transistor 240 is ON, nodes 212 and 224 electrically represent asame point in the circuit. Transistor 270 is ON since gate-sourcevoltage of transistor 270 is greater than the threshold voltage Vt.Therefore, a voltage of 3.3V (from terminal 145) is passed to node 278,thereby causing transistor 260 to be OFF.

Due to the conditions noted above, the circuit of FIG. 2 reduces to thecircuit shown in FIG. 3A. Thus, in the 3.3V mode, capacitors 210 and 220are connected in series between power supply terminal 145 and ground199. The effective decoupling capacitance provided in the 3.3V modeequals (C1*C2)/(C1+C2), wherein C1 and C2 respectively represent thecapacitances of capacitors 210 and 220. Assuming each of C1 and C2equals C, the effective decoupling capacitance provided across terminals145 and 199 equals C/2.

Junction node 224 (or equivalently 212) is DC-biased to power supply 140(through a weak transistor 230), and the voltage at node 224 equals1.8V. Thus, a DC bias of 1.5V (3.3V−1.8V) is provided across capacitor210, and a DC bias of 1.8V (1.8V−0V) is provided across capacitor 220.As noted above, the capacitance of a MOS capacitor is generally afunction of the voltage applied across its terminals. Typically, higherthe voltage (within a maximum limit) higher is the capacitance value ofthe MOS capacitor. The DC bias voltages across capacitors 210 and 220ensure that the capacitance value thus obtained from each of capacitors210 and 220 is sufficiently large. Path 301 in FIG. 3A represents theON-impedance of transistor 230, and is designed to have a large value.

It may be observed from FIG. 3A that the inter-terminal voltages acrosscapacitors 210 and 220 are less than 1.8V. Hence, neither of thecapacitors 210 and 220 is subjected to inter-terminal voltage stressesbeyond the safe limit of 1.8V.

Operation in 1.8 V Mode

In the 1.8V mode, power supply at terminal 145 (Vdds) equals 1.8V, andpower supply at terminal 140 (Vdd) equals 1.8V. Mode signal 135generated by voltage detector cell 170 is a logic one. As a result,transistors 250 and 280 are ON, and transistors 230 and 240 are OFF.Transistor 270 is OFF since gate-source voltage of transistor 270 equals0V. Transistor 260 is ON since source and gate terminals of transistor260 are at 1.8V and 0V respectively.

Due to the conditions noted above, the circuit of FIG. 2 reduces to thecircuit shown in FIG. 3B. Thus, in the 1.8V mode, capacitors 210 and 220are connected in parallel between power supply terminal 145 and ground199. The effective decoupling capacitance provided across terminals 145and 199 in the 1.8V mode equals (C1+C2). Assuming C1 and C2 equal C, theeffective decoupling capacitance equals 2C. A bias of 1.8V is providedacross each of capacitors 210 and 220. Further, the inter-terminalsvoltage across each of capacitors 210 and 220 is equal to 1.8V, andneither of capacitors 210 and 220 is stressed beyond the safe limit of1.8V.

It may be appreciated that the scheme described above enables capacitorsdesigned to withstand lower maximum inter-terminal voltages to be usedto implement a decoupling capacitance cell that can be used in circuitspowered by a power supply voltage greater than the maximuminter-terminal voltage. Further, the techniques described above enablesufficiently large decoupling capacitance values to be obtained in eachof the two modes, i.e., 3.3V mode and 1.8V mode, without subjecting thecapacitors to voltage stresses beyond a safe limit.

Capacitor cells implemented as described above can be used, in general,to provide decoupling capacitance for any circuit (functional circuit)that operates to provide a desired function, and which may need to beoperated (in corresponding operating modes) from one of two possiblepower supply voltages, one power supply voltage being greater than theother. Such functional circuits may additionally need to receive areference voltage equal in value to the lower of the two power supplyvoltages noted above. While noted above as being used for providingdecoupling capacitance for circuits, capacitor cells 160-1 through 160-M(and the capacitance scheme of FIG. 2 in general) can be used innon-decoupling capacitor contexts as well. Further, while thedescription herein is provided with respect to integrated circuit (e.g.,MOS) capacitors, the capacitor cells described herein may be implementedusing other technologies as well (for example, convention parallel platecapacitors).

In the illustrations of FIGS. 1 and 2, though terminals/nodes are shownwith direct connections to various other terminals, it should beappreciated that additional components (as suited for the specificenvironment) may also be present in the path, and accordingly theconnections may be viewed as being electrically coupled to the sameconnected terminals.

The circuit topology of FIG. 2 is merely representative. Variousmodifications, as suited for the specific environment, without departingfrom the scope and spirit of several aspects of the present disclosure,will be apparent to one skilled in the relevant arts by reading thedisclosure provided herein. It should be appreciated that the specifictype of transistors (such as NMOS, PMOS, etc.) noted above are merely byway of illustration. However, alternative embodiments using differentconfigurations and transistors will be apparent to one skilled in therelevant arts by reading the disclosure provided herein. For example,the PMOS transistors may be replaced with NMOS transistors, while alsointerchanging the connections to power and ground terminals.Accordingly, in the instant application, power and ground terminals arereferred to as constant reference potentials, the source (emitter) anddrain (collector) terminals of transistors (through which a current pathis provided when turned on and an open path is provided when turned off)are termed as current terminals, and the gate (base) terminal is termedas a control terminal.

While various embodiments of the present disclosure have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. Thus, the breadth and scope of thepresent disclosure should not be limited by any of the above-describedembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

1. An integrated circuit (IC) comprising: a functional circuit coupledbetween a power supply terminal and a ground terminal of the IC, thefunctional circuit to receive a power supply for operation on the powersupply terminal, wherein the power supply is provided with a firstvoltage value for a first logic-level swing of an output signal of thefunctional circuit, wherein the power supply is provided with a secondvoltage value for a second logic-level swing of the output signal of thefunctional circuit, wherein the first voltage value is greater than thesecond voltage value, and wherein the first logic-level swing is greaterthan the second logic-level swing; and a capacitor cell coupled betweenthe power supply terminal and the ground terminal, the capacitor cellcomprising: a first capacitor and a second capacitor; and a plurality ofswitches operable to couple the first capacitor and the second capacitorin a series arrangement between the power supply terminal and the groundterminal when the output signal is to be provided with the firstlogic-level swing, the plurality of switches operable to couple thefirst capacitor and the second capacitor in a parallel arrangementbetween the power supply terminal and the ground terminal when theoutput signal is to be provided with the second logic-level swing. 2.The IC of claim 1, wherein each of the first capacitor and the secondcapacitor is fabricated as a metal oxide semiconductor (MOS) capacitoraccording to a first fabrication process, wherein each of the firstcapacitor and the second capacitor is designed to withstand a maximuminter-terminal voltage determined by the first fabrication process,wherein the first voltage value exceeds the maximum inter-terminalvoltage.
 3. The IC of claim 2, further comprising a voltage detectorcell to receive the power supply, and to generate a mode signalspecifying whether the power supply has the first voltage value or thesecond voltage value, wherein the mode signal controls the operation ofat least some of the switches in the plurality of switches.
 4. The IC ofclaim 2, wherein the capacitor cell receives a reference voltage,wherein, when the output signal is to be provided with the firstlogic-level swing, the reference voltage is coupled to a junction of thefirst capacitor and the second capacitor.
 5. The IC of claim 1, whereinthe functional circuit is an input/output (I/O) cell, the IC furthercomprising a processor to generate binary data, wherein the I/O cellreceives the binary data and generates the output signal to representthe binary data with a logic-level swing equaling one of the firstlogic-level swing and the second logic-level swing.
 6. A capacitor cellcoupled between a first constant reference potential and a secondconstant reference potential, the capacitor cell comprising: a firstcapacitor and a second capacitor; and a plurality of switches operableto couple the first capacitor and the second capacitor either in aseries arrangement between the first reference potential and the secondreference potential or a parallel arrangement between the firstreference potential and the second reference potential; wherein thecapacitor cell is coupled to provided decoupling capacitance to afunctional circuit, wherein the functional circuit is also coupledbetween the first constant reference potential and the second constantreference potential, the functional circuit to receive the firstconstant reference potential for operation, wherein the first constantreference potential has a first voltage value for a first logic-levelswing of an output signal of the functional circuit, wherein the firstconstant reference potential has a second voltage value for a secondlogic-level swing of the output signal of the functional circuit,wherein the first voltage value is greater than the second voltagevalue, wherein the first logic-level swing is greater than the secondlogic-level swing, wherein the plurality of switches are operable tocouple the first capacitor and the second capacitor in the seriesarrangement when the output signal is to be provided with the firstlogic-level swing, and wherein the plurality of switches are operable tocouple the first capacitor and the second capacitor in the parallelarrangement when the output signal is to be provided with the secondlogic-level swing.
 7. The capacitor cell of claim 6, wherein each of thefirst capacitor and the second capacitor is fabricated as a metal oxidesemiconductor (MOS) capacitor according to a first fabrication process,wherein each of the first capacitor and the second capacitor is designedto withstand a maximum inter-terminal voltage determined by the firstfabrication process, wherein the first voltage value exceeds the maximuminter-terminal voltage.
 8. The capacitor cell of claim 6, wherein a modesignal controls the operation of at least some of the switches in theplurality of switches, the mode signal being generated by a voltagedetector cell, the mode signal specifying whether the first constantreference potential has the first voltage value or the second voltagevalue.
 9. The capacitor cell of claim 6, wherein, when the output signalis to be provided with the first logic-level swing, a reference voltageis coupled to a junction of the first capacitor and the secondcapacitor.